Honestly, for something like dynamat, you shouldn't cover the entire floor or door. But most people do anyway, maybe because they don't know how it works, or, because it ends up looking better since the panels are level and you dont have to use anything else to make the surfaces even.
Dynamat is about weight and vibration absorption, adding weight to panels to stop resonance/vibrations but not noise, as its not made for that. You only need enough to stop the vibration. You see people wrapping the entire floor and cargo area in it, when they only need a few sheets placed in specific places, usually in the middle of a large metal panel to stop any unwanted vibrations.
For road noise or other noise, like wind hitting through the top, you need something like the liner, dynaliner I believe it is? Its some kind of rubber/foam/neoprene kind of thing. Its actually designed to stop sound and to be used along dynamat and also works as thermal insulation if I recall.